This invention relates generally to the cooling of charged-particle-transparent windows used in charged particle accelerator systems and, in particular, to a phase transition cooled window for broad beam electron guns.
The operation of a charged particle accelerator system requires that the particle-emitting cathode be housed in a vacuum-tight housing. Typically, on the wall of the housing opposite the enclosed cathode, there is disposed a particle-attracting anode, which is transparent to such particles while impervious to gas. The vacuum within the housing is thereby maintained while permitting the accelerated particles to be emitted from the accelerator. The particles emitted from the cathode are attracted and accelerated toward the anode, and upon reaching the anode, pass through the particle-transparent anode window to the exterior of the accelerator. In practice, a window which is perfectly transparent to such particles is not realizable. Therefore, the passage of the particles through the window results in some interaction by the particles with the window material. This results in the generation of heat. When particle beam intensities are very high, the heat generated in transparent windows is also very high, leading to possible degeneration of the window material. It is, therefore, desirable to devise some means for cooling these transparent windows.
Desirable characteristics of any cooling means include minimal interference by the cooling apparatus with the particle beam, as well as capacity to remove large quantities of heat.
In the past, attempts at developing such cooling systems have been directed toward the cooling of the periphery of the window, circulation of a gas over the surface of the window, and circulation of a liquid, without phase-over, over the window. In all of the above, the property of thermal conduction is utilized. That is, heat is carried away by electron conduction in metals and by vibration transfer in gases and liquids, without a phase-change in the coolant materials.
When, as above, the periphery of the window is cooled, heat is transferred by electron conduction in the window material. The quantity of heat which may be removed is determined by the heat transfer characteristics of the anode window material. In practice, the above method has proved to be unsatisfactory, often leading to large temperature gradients across the surface of the window and not infrequent disintegration of the anode window material due to excess heat build up.
In a system using gases to take up and carry away heat from the window, the low mass density of the gases satisfies the non-interference requirement above. However, the amount of heat which can be carried away by a gas cooling system is small.
In the liquid based cooling system, heat is transferred to the liquid and carried away. While the heat-carrying capacity of liquids is much higher than that for gases, liquids have higher areal mass densities and, hence, higher degrees of interference with the electron beam. Reference is made to R. C. Marker, et al. U.S. Pat. No. 3,105,916, wherein a liquid based cooling system is described.